;-------------------------------------
; function dx/dt (called by the Dst model)
;-------------------------------------

FUNCTION d1x_pro, t, x, P1

tau = 2.4*exp(9.47/(4.69 + P1))  ; [hours]

if (P1 le 0.49) then begin
  Q = 0.
endif else begin     
  Q = -4.4 *(P1-0.49) ; [nT/hours]
endelse

d1x = Q - x*(tau^(-1.0)) ; [nT/hours]
        
return, d1x

END